1. Field of the Invention
The present invention relates to an antenna technique, and more particularly, to a technique for downsizing an antenna.
2. Description of the Related Art
An RFID (Radio Frequency Identification) technique is a technique for identifying a data carrier by using a radio wave without contact. With this technique, a tag (RFID tag) in which an IC chip and an antenna are embedded is attached to an object or a person, a device called an RFID reader/writer, and the RFID tag make a communication by using a radio wave, and the RFID reader/writer reads information stored in the IC chip (RFID tag IC), whereby the object or the person is identified.
FIG. 1 shows a first example of an antenna available to an RFID tag. This antenna is a fundamental dipole antenna which is formed with antenna lines 101, and the entire length of which is a ½ wavelength, and the RFID tag is configured by connecting this antenna to an RFID tag IC 10.
Additionally, FIG. 2 shows a second example of an antenna available to an RFID tag. This antenna is a fundamental loop antenna which is formed with an antenna line 102, and the entire circumference of which is 1 wavelength, and the RFID tag is configured by connecting this antenna to an RFID tag IC 10.
As described above, the RFID tag becomes very large if the fundamental antenna such as a dipole antenna, a loop antenna, etc. is used unchanged. Therefore, the downsizing of the antenna is demanded for the RFID tag.
A number of techniques for downsizing an antenna are proposed conventionally.
For example, FIG. 3 shows a third example of an antenna available to an RFID tag. This antenna is downsized by bending a pair of antenna lines 101, which forms the dipole antenna according to the first example shown in FIG. 1, in a shape meandering at a right angle.
Furthermore, FIG. 4 shows a fourth example of an antenna available to an RFID tag. This antenna is downsized by bending the loop-shaped antenna line 102, which configures the loop antenna shown in FIG. 2, along a Hilbert fractal pattern.
Still further, FIG. 5 shows a fifth example of an antenna available to an RFID tag. This antenna is downsized by bending the loop-shaped antenna line 102, which configures the loop antenna shown in FIG. 2, along a Koch fractal pattern. The fractal means that a portion of a graphic is self-similar to the entire graphic, for example, as shown in FIG. 6A depicting the partial view of a pattern of FIG. 5, and FIG. 6B depicting the outline of the pattern. Namely, a straight line portion of a large shape is replaced with a shape similar to the entire shape as in a relationship of replacing a pattern 102a with a pattern 102b, and a relationship of replacing the pattern 102b with a pattern 102c in FIG. 6B. Therefore, a longer path length can be obtained by implementing the shape of the antenna line 102 to be a fractal pattern.
As described above, the technique using a conductor line, which is made to meander, as an antenna element is conventionally carried out.
For example, Patent Document 1 discloses a technique for forming a loop antenna by arranging 2 coils upright on a ground conductor, and by connecting the 2 coils with a meandering line or plate conductor.    Patent Document 1: Japanese Unexamined Published Patent Application No. 2001-284935
Additionally, for example, Patent Document 2 discloses a technique for arranging and bending a pair of zigzag antenna elements, which configures a dipole antenna, along the inner wall surface of a housing.    Patent Document 2: Japanese Unexamined Published Patent Application No. 2000-349526
Furthermore, for example, Patent Document 3 discloses, for example, as shown in FIG. 7, a loop antenna the line conductor of which is bent in the shape of a snowflake.    Patent Document 3: U.S. Pat. No. 6,603,440
An antenna comprised by an RFID tag is not only demanded to be small as described above, but also desired to reduce an antenna gain in order to obtain a long communication distance. Additionally, for an RFID tag, so-called a passive tag, which does not comprise a battery within the RFID tag itself, and uses a radio wave from an RFID reader/writer as a power source for its operations, an antenna having high impedance is demanded to obtain a high power source voltage.
However, if the antenna is downsized by making a conductor line, which is an antenna element, simply meander, degradations in a gain and impedance are caused in contradiction to the above described demands.